1. Field of the Invention
The field of the present invention relates generally to AC motor direction control and switching circuitry therefor. There are numerous applications in which the rotational direction of an AC motor is required to be reversed. One such application is impact printing, in which a printing head such as a matrix printer under computer control prints data through use of an inked ribbon upon an ink receiving medium. Either one or two motors wind ribbon on storage spools during printing to continuously expose fresh ink to the printer, which requires reversing the ribbon direction at the ends thereof by reversing the direction of rotation of the motor or motors driving the spools. Switches are actuated to detect ribbon end points which in turn cause motor current reversing switching to occur. A problem in systems of this type is that the making and breaking of the ribbon end point detecting and motor reversing switches causes arcing which in turn generates RFI and other noise and transients. This noise in systems of the prior art interferes with the low level logic voltages from the computer control which enable the printing head with consequent printing errors and system breakdowns.
2. Description of the Prior Art
In the prior art, elimination of RFI caused by switching transients has heretofore been attempted by means of large capacitors connected across the switches to reduce large rates of changes of voltage when switching occurs with high voltage across and current through the switch. In the present invention, switching always occurs when no such voltages or currents are present across or through the switches, thereby obviating the need for such prior art capacitors which are both expensive and potentially dangerous due to their high discharge potentials. Various other approaches taken by the prior art include the current reversal system of U.S. Pat. No. 3,611,093, in which current is rapidly reversed through an impedance element by establishing resonance therewith when current is interrupted. Another technique of the prior art is described by U.S. Pat. No. 3,432,736 in which reversal is accomplished by connecting an AC source across half of an autotransformer, with each of the autotransformer windings connected to a separate triac. The other triac terminals are connected to one phase of the motor field windings. One triac is energized to connect one end of the autotransformer to the field winding with each end being 180.degree. out of phase.